CB, I really think your book is misinforming, or whoever wrote it didn't correctly baffle the filter, because I haven't ever heard of UGF's being a problem for long finned fish. I've tried them, and I haven't ever had any problems with fin ripping from them, or strong outflow buffeting my fish and pushing them everywhere.
Now if they were using a power head hooked up to some insane monster UG in a 2 gallon aquarium and had something like a guppy in there I could understand that then. My guess is that either it was a typo in your book, the writer had a bad bit of luck OR the writer had something else in their aquarium that was shredding the fins of their fish.
You cannot believe everything you read even if its from some top amazing author. It's better to go by other peoples first hand experiences, people you know are real rather than just some random author of a book. But to each his or her own, if that's what you believe then that's what you believe. We are just telling you, all of us are telling you that none of us have ever, not once had any issues with an UG filter with our long finned fish.
Keep in mind, mine is powered by an airlift tube as opposed to a power head. Therefore, there is minimal outflow from the filter and even less going in due to the surface area of the grates under the gravel. I have 4 platys and my betta living happily with the flow. (10 gal, 9.5 inches of fish)
Whoops got mixed up it says just bad for Guppies. "Breeders report that after using an undergravel filter for week or two, you will be able to watch the males' tail almost fall apart, due to a high bacteria count in their water. Although may prove useful for a fish store, they are not a good choice for a serious guppy breeder." This guy raises and competes guppiess in shows.
But since he's a breeder that shows guppies his definition of "falling apart" may be far different from ours. He wants absolute perfection so he may have been exaggerating by our standards of a normal healthy guppy.
I've never heard of good bacteria destroying a fish's fins, it sounds more like he had finrot and wasn't taking care of his tank like he should have, especially since he had only been using it for 2 weeks.
CB, he is a breeder and a shower I'm sure he knows what he's doing in that department but that does not mean he knows everything about UG and HOB filters.
Him saying that the UG had a large amount of bacterial buildup means nothing since all filters encourage this. Saying the UG is bad because it's doing an amazing job at what it's meant to do is preposterous.